All cultivated forms of lettuce belong to the highly polymorphic species, Lactuca sativa, which is grown for its edible head and leaves. As a crop, lettuces are grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, scorzonera, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of pests and diseases such as downy mildew (Bremia lactucae). This disease result in millions of dollars of lost lettuce crop throughout the world every year. Downy mildew (Bremia lactucae) is highly destructive of lettuce grown at relatively low temperature and high humidity. Downy mildew is caused by a fungus, Bremia lactucae, which can be but is not limited to one of the following strains: NL1, NL2, NL4, NL5, NL6, NL7, NL10, NL12, NL13, NL14, NL15, NL16, B1:17, B1:18, B1:20, B1:21, B1:22, B1:23, B1:24, B1:25, B1:26, B1:27, B1:28 (Van Ettekoven, K. et al., “Identification and denomination of ‘new’ races of Bremia lactucae,” In: Lebeda, A. and Kristkova, E (eds.), Eucarpia Leafy Vegetables, 1999, Palacky University, Olomouc, Czech Republic, pp. 171-175; Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants); IBEB press release “ New race of Bremia lactucae B1:27 identified and nominated”, May 2010; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), “New race of Bremia lactucae B1:28 identified and nominated”, March 2011), Ca-I, Ca-IIA, Ca-IIB, Ca-III, Ca-IV (Schettini, T. M.; Legg, E. J.; Michelmore, R. W., 1991. Insensitivity to metalaxyl in California populations of Bremia lactucae and resistance of California lettuce cultivars to downy mildew. Phytopathology 81(1). p. 64-70.), Ca-V, Ca-VI, Ca-VII, Ca-VIII (Michelmore R. & Ochoa. O. “Breeding Crisphead Lettuce.” In: California Lettuce Research Board, Annual Report 2005-2006, 2006, Salinas, California, pp. 55-68). Also new strains appear, which are overcoming resistance gene combinations in present varieties. An example is strain FR10.021 which is overcoming the Bremia-resistance in the variety “Expedition”. New strains can be described by a sextetcode based on resistance test results on a set of 25 publicly available lettuce lines (IBEB press release May 2010—the pdf link is dated May 3, 2010 and is available on the website of Plantum NL). FR10.021 has sextetcode 63-31-62-03.
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the fungus progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Lettuce mosaic virus (LMV) mainly infects lettuce seeds, which is the primary way that the virus is introduced to lettuce in the fields, but also can infect numerous crops and weeds, thereby creating reservoirs of the virus. LMV also can be vectored by aphids, which spread the virus within a lettuce field and introduce it into lettuce fields from infected weeds and crops outside the field.
Symptoms of lettuce mosaic virus vary greatly. Leaves of plants that are infected at a young stage are stunted, deformed, and (in some varieties) show a mosaic or mottling pattern. Such plants rarely grow to full size; head lettuce varieties infected early fail to form heads. Plants that are infected later in the growth cycle show a different set of symptoms. These plants may reach full size, but the older outer leaves turn yellow, twisted, and otherwise are deformed. On head lettuce, the wrapper leaves often will curve back away from the head and developing heads may be deformed. In some cases brown, necrotic flecks occur on the wrapper leaves.
Of the various species of aphids that feed on lettuce, the currant-lettuce aphid (Nasonovia ribisnigri) is the most destructive species because it feeds both on the leaves of the lettuce as well as deep in the heart of the lettuce, making it difficult to control with conventional insecticides. The lettuce aphid feeds by sucking sap from the lettuce leaves. Although direct damage to the lettuce may be limited, its infestation has serious consequences because the presence of aphids makes lettuce unacceptable to consumers.
Romaine lettuce should provide a product at harvestable stage, which is accepted by processing industry and/or consumers. Therefore the harvestable product should not have tipburn, and it should have thick, strongly blistered leaves with a dark green outer leaf color. Further it should be sufficiently headed, and as a result of this heading provide sufficient yellow-colored heartleaves.
Mature romaine lettuce is mainly harvested by hand. This harvesting process is labor intensive, and relatively laborer-unfriendly. As it getting more and more costly to hire laborers that want to work outdoors in the field, close to the ground and under all kinds of adverse weather conditions, there is strong interest in the lettuce industry to mechanize this manual harvesting process. However, until now the lack of uniform quality among mature plants is a major problem for mechanical harvest. Poor quality of basal leaves, internal breakdown of heartleaves due to tipburn, fringe burn of leaf margins and fast bolting are the main quality problems. If one would like to mechanize the harvest process, these quality problems result in additional hand sorting, which is costly, and/or a very low net yield, if the plant parts with a high risk of quality problems are mechanically discarded. The latter approach is chosen by some producers that produce romaine hearts, i.e. the intact yellow-green heart of the romaine lettuce without the green outer leaves. For producers that produce romaine lettuce for processing, i.e. pre-washed and pre-cut leaves, this is not feasible because of three major constraints.
The first is the requirement for a mix of green and yellow leaves, which requires the preservation of the undamaged green outer leaves.
The second constraint is caused by the use of mechanically-driven horizontal knives that easily damage the hollow main veins of romaine leaves and cut halfway through the prostrate, round-shaped, lower outer leaves, which results in wide cut surfaces. Cutting damage on a leaf with a hollow main vein is often not restricted to the cut surface but bruising extends into the leaf along the main vein.
The third constraint is the requirement for a leaf to have a good shelf life. This means the leaves should get through the washing and drying process in the factory without bruising or breaking The leaves should be sufficiently thick and tough to avoid wilting. In addition, the leaves should be strongly blistered to avoid sticking to each other and to the plastic of the packaging, which increases local humidity on the leaf and reduces local respiration.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.